LU-8964 clio: Parallelize generic I/O

LU-8964 clio: Parallelize generic I/O

Add parallel version of cl_io_loop() function which use information
about stripes from LOV layer and process them in parallel.
This feature is disabled by default. To enable it you should run
"lctl set_param llite.*.pio=1" command.

IOR results on KNL for:
access             = file-per-process
ordering in a file = sequential offsets
ordering inter file= no tasks offsets
clients            = 1 (1 per node)
repetitions        = 1
blocksize          = 128 GiB
aggregate filesize = 128 GiB

xfsize	pio	Write	Read
16	none	170.46	372.12
16	off	370.46	926.53
16	on	668.49	899.55
32	off	368.75	908.95
32	on	469.54	987.64

IOR results on Broadwell Xeon for:
access             = file-per-process
ordering in a file = sequential offsets
ordering inter file= no tasks offsets
clients            = 1 (1 per node)
repetitions        = 1
xfersize           = 16 MiB
blocksize          = 128 GiB
aggregate filesize = 128 GiB

pio	Write	Read
none	1419.80	1277.88
off	1348.98	2245.84
on	 990.76	2320.08

The scalability IOR results on other Broadwell Xeon for:
access             = file-per-process
ordering in a file = sequential offsets
ordering inter file= no tasks offsets
repetitions        = 1
xfersize           = 4 MiB
blocksize          = 8 GiB

Threads	pio	Write	Read
 32	off	9358.38	2649.28
 32	on	9147.14	2677.44
 64	off	8538.65	2811.05
 64	on	8944.19	2908.44
128	off	7978.61	2937.03
128	on	8613.91	2928.44

The numbers are in ‘MB/s’

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: Id028faba1726fb377d0e903e8b8095d5ea9d1ee2
Reviewed-on: https://review.whamcloud.com/26468
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>